Beam hardening is a phenomenon in medical imaging that can influence the clarity and accuracy of diagnostic images, particularly those produced by X-ray computed tomography (CT) scans. It occurs when an X-ray beam passes through an object, such as human tissue, causing a change in the beam’s energy composition. Understanding beam hardening is important because it can lead to image artifacts, which are distortions that might obscure important medical details. These distortions can affect the ability to accurately interpret scans and make informed clinical decisions.
Understanding X-ray Energy
X-ray beams used in imaging systems are not composed of a single, uniform energy. Instead, they are “polychromatic,” meaning they contain a range of different X-ray energies. Within this polychromatic beam, X-rays are described as “softer” or “harder” based on their energy. Softer X-rays possess lower energy, while harder X-rays have higher energy. This energy spectrum is a characteristic of the X-ray beam generated by the imaging equipment.
The Phenomenon of Beam Hardening
Beam hardening occurs because different X-ray energies interact differently with matter. As a polychromatic X-ray beam travels through an object, the lower-energy X-rays are absorbed more readily than the higher-energy X-rays. This selective absorption means that the “softer” X-rays are filtered out more quickly. Consequently, the average energy of the remaining X-ray beam increases as it penetrates deeper into the object. This increase in the mean beam energy is what is referred to as the beam becoming “harder.” The composition of the material and the total distance the X-ray travels through it both influence this effect.
How Beam Hardening Affects Images
The change in the X-ray beam’s energy composition as it passes through the body can lead to specific visual distortions, known as artifacts, in the final image. These artifacts can obscure fine details and potentially reduce the diagnostic reliability of the scan. Two common types of these artifacts are “cupping artifacts” and “streaking artifacts.”
Cupping Artifacts
Cupping artifacts typically appear as a darker region in the center of an object that should otherwise appear uniform in density. This happens because the X-rays passing through the thicker middle section of an object harden more than those passing through the edges, leading to lower measured CT numbers in the center.
Streaking Artifacts
Streaking artifacts manifest as dark bands or streaks between two dense objects in the image, such as bones or metallic implants. These streaks arise from the X-ray beam hardening at different rates depending on the angle of the X-ray source and detector around the patient.
Strategies for Minimizing Beam Hardening
Several strategies are employed in X-ray imaging to minimize beam hardening.
Filtering
One common approach involves “filtering” the X-ray beam before it reaches the patient. This is achieved by placing a thin sheet of metal, often aluminum or copper, in the beam path to absorb the lowest energy photons. This “pre-hardens” the beam.
Software Correction Algorithms
Software correction algorithms are applied during or after the image reconstruction process. These sophisticated computer programs analyze the acquired data and computationally adjust for the nonlinear attenuation caused by beam hardening. Many modern CT scanners incorporate built-in corrections to reduce common artifacts like cupping.
Increasing Kilovoltage Peak (kVp)
Increasing the kilovoltage peak (kVp) is a direct way to reduce beam hardening by generating a higher average energy X-ray beam. When the X-ray tube voltage is set higher, the resulting X-rays are inherently more penetrating, which can lessen the relative impact of beam hardening. However, this method can sometimes lead to reduced contrast in low-density areas of the image.
Dual-Energy CT (DECT)
Dual-energy CT (DECT) is an advanced technique that uses two different X-ray energy levels to acquire data. By collecting information at both a high and a low energy, DECT systems can differentiate between various materials more effectively and create “virtual monochromatic” images. These virtual images simulate what the scan would look like if a single-energy X-ray beam were used, which can significantly reduce or even inherently eliminate beam hardening artifacts.